1. Field of the Invention
The present invention generally relates to a development device, a process cartridge, and an image forming apparatus such as a copier, a printer, a facsimile machine, or a multifunction machine capable of at least two of these functions and which includes the development device.
2. Discussion of the Background Art
In general, electrophotographic image forming apparatuses, such as copiers, printers, facsimile machines, or multifunction devices including at least two of those functions, etc., include a latent image carrier on which an electrostatic latent image is formed and a development device to develop the latent image with developer. In electrophotographic images forming apparatuses, two-component developer consisting essentially of toner and carrier particles is widely used.
Development devices using two-component developer (hereinafter “two-component development devices”) are typically configured to include a developer carrier (e.g., development roller) to carry the developer thereon, multiple developer transport chambers (e.g., a developer containing part) extending in a longitudinal direction of the development device, and multiple developer transport members to transport the developer within the multiple developer transport chambers, thus circulating the developer inside the development device. The multiple developer transport chambers include a developer supply chamber, from which the developer is supplied to a circumferential surface of the developer carrier, and a collection chamber, to which the developer that has been used in image development is collected from the circumferential surface of the developer carrier. These developer transport chambers are separated partially or entirely by a wall or partition.
At least two of the multiple developer transport members are often arranged vertically because this arrangement can make the development device horizontally compact compared with an arrangement in which the multiple developer transport members are arranged horizontally. In particular, this arrangement is widely used in tandem-type multicolor image forming apparatuses that include multiple development devices arranged horizontally.
Multiple magnets are fixed inside the developer carrier to form corresponding multiple magnetic poles around the developer carrier. The multiple magnetic poles include an attraction pole to attract the developer to the circumferential surface of the developer carrier, a development pole that contributes to image development, and a release pole that contributes to separating the developer from the developer carrier after image development.
In two-component development devices, fresh toner is supplied to the development device as the toner therein is consumed in image development. The supplied toner is mixed with the developer in the development device by the developer transport member (e.g., a transport screw), and then the mixed developer is partly supplied to the circumferential surface of the developer carrier. While the developer carrier rotates, a doctor blade serving as a developer regulator adjusts the amount of the developer carried on the surface of the developer carrier, and then the toner in the two-component developer is adhered to a latent image formed on an image carrier in a development range or area where the development roller faces the image carrier.
In such two-component development devices, because the concentration of toner in the developer that has been used in image development (hereinafter “used developer”) is lower than that which has not been used in image development, image density can be reduced or become uneven if such used developer is again supplied to the developer carrier.
In view of the foregoing, several approaches, described below, have been tried to prevent the used developer from being supplied to the developer carrier.
For example, in certain known development devices, the developer supply chamber (e.g. a first developer transport chamber) is positioned above the developer collection chamber (e.g., second developer transport chamber). The developer transport member (e.g., a first developer transport member) disposed in the developer supply chamber supplies the developer to the development roller at a position corresponding to the attraction pole (hereinafter “attraction position”) while transporting the developer within the developer supply chamber longitudinally within the developer carrier of the development device. Then, the developer is separated from the development roller at a position corresponding to the release pole (hereinafter “release position”) and is collected in the developer collection chamber in which the developer transport member (e.g., a second developer transport member) transports the developer in a direction opposite the direction in which the first developer transport member transports the developer (hereinafter “developer circulation direction”).
Although in the above-described configuration the first developer transport chamber and the second developer transport chamber are separated, a downstream portion of the first developer transport chamber and an upstream portion of the second developer transport chamber in the developer circulation direction communicate with each other through a first communication portion, and the developer falls under its own weight through the first communication portion from the first developer transport chamber to the second developer transport chamber. Similarly, an upstream portion of the first developer transport chamber and a downstream portion of the second developer transport chamber in the developer circulation direction communicate with each other through a second communication portion.
The mixture including the developer separated from the development roller, that which has fallen through the first communication portion, and that transported from the upstream portion of the second developer transport chamber accumulates in the downstream portion of the second developer transport chamber in the developer circulation direction and then is pushed up by the pressure of the accumulation through the second communication portion to the upstream portion of the first developer transport chamber.
Vertically arranging the developer supply chamber and the developer collection chamber can better inhibit the used developer from being resupplied to the development roller compared with the arrangement in which multiple developer transport members are arranged horizontally. Accordingly, unevenness in the amount of toner forming a toner image on the image carrier can be reduced.
However, in this known configuration, it can still happen that the developer used in image development and then separated form the development roller at the release position might be again carried on the development roller by rotation of the second developer transport member, a phenomenon which is hereinafter referred to as “resupply of used developer”. If resupply of used developer occurs, the used developer having a reduced toner concentration is supplied to the development roller, making the image density uneven.
Occurrence of resupply of used developer is more frequent in the downstream portion of the second developer transport chamber, adjacent to the second communication portion, where the amount (height) of accumulated developer is greater. In particular, resupply of used developer occurs more often when the dimensions of the second developer transport chamber are reduced to make the development device more compact. More specifically, when the dimensions of the second developer transport chamber are smaller, the developer occupies proportionally more of the second developer transport chamber, and accordingly, resupply of used developer can occur more easily. Additionally, unevenness in the image density can also be significant when the second communication portion is disposed closer to the development range, because the used developer carried again on the development roller is immediately used in image development if resupply of used developer occurs.
In another approach, the wall dividing the two vertically arranged developer transport chambers is extended to an contact the development roller and functions as a developer separation member to remove the developer mechanically from the circumferential surface of the development roller.
Although this known configuration is successful in removing the resupplied used developer mechanically from the development roller even if the developer separated from the development roller at the release position is again carried on the development roller, the developer separation member being in contact with the development roller can increase the wear on the development roller as well as the driving torque of the development device, which is not desirable.
In view of the foregoing, in development devices including two vertically arranged developer transport chambers in which developer is transported in the longitudinally within the development device, there is a need to prevent carrying over of the developer without increasing the wear of the developer carrier, which the known image forming apparatuses fail to do.